linux-kernel - Re: [PATCH v2 14/14] bpf,x86: Respect X86_FEATURE

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Message-ID: <20211021000753.kdxtjl3nzre2zshb@ast-mbp.dhcp.thefacebook.com>
Date:   Wed, 20 Oct 2021 17:07:53 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, jpoimboe@...hat.com, andrew.cooper3@...rix.com,
        linux-kernel@...r.kernel.org, ndesaulniers@...gle.com,
        bpf@...r.kernel.org, daniel@...earbox.net
Subject: Re: [PATCH v2 14/14] bpf,x86: Respect X86_FEATURE_RETPOLINE*

On Wed, Oct 20, 2021 at 12:44:56PM +0200, Peter Zijlstra wrote:
> +
> +	if (cpu_feature_enabled(X86_FEATURE_RETPOLINE_AMD)) {
> +		EMIT_LFENCE();
> +		EMIT2(0xFF, 0xE0 + reg);
> +	} else if (cpu_feature_enabled(X86_FEATURE_RETPOLINE)) {
> +		emit_jump(&prog, reg_thunk[reg], ip);
> +	} else

One more question.
What's a deal with AMD? I thought the retpoline is effective on it as well.
lfence is an optimization or retpoline turned out to be not enough
in some cases?